The C standard helpfully permits enums with a small range of values to
be short. This effect is not desirable in an API because it means that
adding new values (new flags, say) may cause ABI incompatibility.

Add some `sizeforce' members to relevant enums in the API. In none of
the three cases do we expect this to actually cause an ABI break on
actual practical platforms, because the values we choose are within
the same power-of-two bitwidth.

The small max value of adns_initflags is unfortunate. But we can't
change it without actually breaking the ABI on actual existing
platforms. We can put off biting that bullet until we run out of
initflags - there are still three flags remaining.

This patch is not expected to have any practical effect with existing
platforms and compilers but it makes the code more correct and less
likely to be misoptimised by a future aggressive compiler.